A major market for optical systems and devices is portable electronics equipment such as cellular telephones, pagers, portable computers and the like. Generally, it is desirable for the information displays used in devices of this type to be compact with low power requirements, inexpensive and yet capable of providing a high quality image. As larger and more complex messages are being sent to remote units and more information is being displayed on portable computers, the ability of the display to present information to the user in a readable format is becoming more challenging. Prior art systems for displaying large amounts of information typically comprise either a large display that is directly viewed such as is used in a conventional portable laptop computer, or comprise a small image source with high magnification optics which create a high magnification virtual display. The major problem with a direct view system is that it greatly limits the minimum size of the portable electronics on which it is used. This is because the image must be large enough for an operator to read and understand the information being displayed. Thus, for example, in order to display 40 lines of information at a pitch of 10 characters per inch, a conventional laptop computer display must be on the order of 20 centimeters by 25 centimeters. Such a display would clearly be too large to be incorporated into most portable communications devices such as pagers, cellular telephones and the like.
In typical designs of virtual image optical systems, the elements composing the magnification optics typically have surfaces with significant curvatures, in order to achieve the desired magnification levels in a relatively compact package. These elements typically have small numerical apertures which increases the illumination requirements of the display device. Further, the steep curvature of these optical elements produce significant spherical and chromatic aberrations which must be corrected by additional optical elements thereby increasing the size, cost and power requirements of the optical system.
Radially gradient index (GRIN) glass cylinder lenses with flat surfaces at both ends have been suggested as possible substitutes for homogeneous glass lenses in an optical system. The advantage of the GRIN lens is that the numerical aperture of a flat cylinder lens is substantially greater than an equally sized homogeneous glass lens and a GRIN lens produces fewer monochromatic and chromatic aberrations. Accordingly, GRIN lens elements have found substantial use in optical systems to correct monochromatic and chromatic lens aberrations.
It has been suggested that axially gradient index of refraction glass lenses such as axial GRADIUM manufactured by LightPath Technologies, Inc. may also be suitable for fabricating lenses with reduced aberrations and as light coupling elements. For example, in U.S. Pat. No. 5,936,777 to Dempewolf, there is disclosed an optical coupler comprising a single lens having two axially gradient index of refraction elements joined along their respective high refractive index surfaces that are used as a coupler having an optical axis normal to the index of refraction gradient.
What has not been taught or suggested by the prior art, however, is that a biaxially gradient index of refraction lens having an optical axis parallel to the refractive index gradient can be utilized as a highly efficient, compact, magnification lens suitable for use in magnifying, for example, a miniature display element contained in a portable electronic device.